The subject matter of this Application is related in part to that of the following four U.S. patent applications Ser. No. 09/090,989, filed Jun. 4, 1998; Ser. No. 09/136,353, filed Aug. 18, 1998; Ser. No. 09/317,527, filed May 24, 1999; and Ser. No. 09/317,695, filed May 24, 1999. Said applications relate generally to molecular fluorine lasers with single line selection. U.S. application Ser. No. 09/317,527 also relates generally to line narrowing of the selected emission line and is hereby incorporated by reference in its entirety.
The invention relates to a means and method of broadening the spectrum of a single line output beam of a molecular fluorine laser and thereby decreasing the beam""s temporal coherence length.
Line-narrowed excimer lasers used for microlithography must provide output beams having certain spectral and temporal coherence properties according to the requirements of lithographic imaging systems. The design of the projection optics sets an upper limit to the spectral linewidth of the excimer laser. The design of the illumination optics determines the upper limit of the coherence length and thus the lower limit of the spectral bandwidth of the excimer laser. When the coherence length is longer than a specified minimal length (i.e., the spectral bandwidth is narrower than allowed by the design of the illumination optics), the image on the wafer shows random modulation due to the effect of speckle.
Narrowing of the linewidth of excimer lasers is achieved most commonly through the use of a wavelength selector consisting of prisms and diffraction gratings. In a molecular fluorine laser operating at the wavelength of approximately 157 nm, use of reflective diffraction gratings is limited due to their low reflectivity and the high oscillation threshold inherent with this type of laser.
However, F2 excimer lasers emit light at two or three or more spectral lines, where each line is relatively narrow (on the order of picometers). The present invention provides a means of achieving upper and lower limits on spectral bandwidth output by first, selecting one of these lines and, second, increasing the spectral bandwidth of the selected single line by appropriate means. The increase in the spectral bandwidth is equivalent to a decrease of temporal coherence, thus reducing speckle effects.
U.S. Pat. No. 5,856,991 describes the use of an optical etalon as an outcoupler, which etalon serves at the same time to line-narrow KrF or ArF excimer lasers. U.S. Pat. 4,977,563 describes the use of a pressure-tuned optical etalon. V. N. Ischenko, S. A. Kochubei, A. M. Razhev, xe2x80x9cHigh-power efficient vacuum ultraviolet laser excited by an electric dischargexe2x80x9d, Soviet Journal of Quantum Electronics, v.16, pp.707-709, 1986 show molecular fluorescence spectra of fluorine.
It is an object of the present invention to select an emission line of a molecular fluorine laser and to provide a means and method of broadening the spectrum of the single output beam line and thereby decrease the laser beam""s temporal coherence length.
The detailed optical means and method for doing this are described below in relation to each of the various embodiments of the invention.
The spectral bandwidth of a selected emission line is dependent in part on the laser chamber pressure and the reflectivity of the outcoupling mirror, if present. Accordingly, the embodiments below function better when the laser chamber pressure is relatively high and the reflectivity of any outcoupling mirror is also relatively high. As our experience confirms, the spectral bandwidth of the selected laser line increases with increasing chamber pressure; therefore, for all embodiments of the present invention, the laser chamber pressure should preferably be above 2.5 bar.
From the fluorescence spectra of molecular fluorine F2 published elsewhere (see e.g. V. N. Ischenko, S. A. Kochubei, A. M. Razhev, xe2x80x9cHigh-power efficient vacuum ultraviolet laser excited by an electric dischargexe2x80x9d, Soviet Journal of Quantum Electronics, v. 16, pp.707-709, 1986), it is known that the gain curve for a single line is rather broad in spectral width. Conventionally, excimer lasers employ low reflectivity outcoupling mirrors (reflectivity  less than 8%) because of the high gain of the excimer. This is also the case for the F2 excimer laser. Laser oscillations can only occur in the part of the spectral gain curve in which the gain exceeds the losses. Therefore, increasing the reflectivity of the outcoupling mirror will lead to a broader spectral bandwidth for the single line F2 laser output. Accordingly, for the embodiments of the present invention which have an outcoupling mirror, the reflectivity of the outcoupling mirror should preferably be above 8%. With proper selection of the outcoupler reflectivity and the laser chamber pressure, the spectral bandwidth and therefore the coherence length of the output can be adjusted to meet the requirements of microlithography.
In the first embodiment, line selection is achieved through the use of a dispersive prism. Further line shaping, i.e., spectral broadening of the line, is achieved by using a high reflectivity outcoupling mirror (reflectivity R between 8% and 90%) and a gas pressure in the laser chamber above 2.5 bar.
In the second preferred embodiment, line selection is again achieved through the use of a dispersive prism. Further line shaping, i.e., spectral broadening of the line, is achieved by using a high reflectivity mirror at the other side of the cavity (reflectivity R between 90% and 100%) and a gas pressure in the laser chamber above 2.5 bar. Outcoupling of the laser beam is achieved by reflection at one surface of the prism. The design of the prism, i.e., the angles of the prism as well as the coatings on its surface, can be used to vary the outcoupling ratio.
In the third embodiment, line selection is also achieved through the use of a dispersive prism. Further line shaping, i.e., spectral broadening of the line, is achieved by using an etalon as an outcoupling mirror which is tuned antiresonant to the selected single laser line. Gas pressure in the laser chamber is again above 2.5 bar.
In the fourth preferred embodiment, line selection is again achieved through the use of a dispersive prism. Further line shaping, i.e., spectral broadening of the line, is achieved by using an etalon as a reflecting mirror which is tuned antiresonant to the selected single laser line. Gas pressure in the laser chamber is again above 2.5 bar. Outcoupling of the laser beam is achieved by reflection at the one surface of the prism.
In the fifth embodiment, an etalon provides line selection and, therefore, the use of a prism is not necessary. In addition, the etalon acts as an antiresonant outcoupling mirror which suppresses the center of the selected single laser line. The advantage over the first four embodiments is simplicity and a reduction of elements in the resonator which leads to reduced optical losses and increased lifetime. However, the suppression of the second line may be less efficient.
In the sixth embodiment, line selection is achieved through the use of a high reflectivity etalon at one side of the resonator. Further line shaping, i.e., spectral broadening of the line, is achieved by using a second etalon as an outcoupling mirror which is tuned antiresonant to the selected single laser line. Gas pressure in the laser chamber is again above 2.5 bar.
In the seventh embodiment, line selection is achieved through the use of a high reflectivity etalon at one side of the resonator. Further line shaping, i.e., spectral broadening of the line, is achieved by using a higher reflectivity outcoupling mirror (reflectivity R between 8% and 90%) and a gas pressure in the laser chamber above 2.5 bar.